Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Asymmetric loading across these rotors occurs due to vertical and horizontal wind shears, yaw misalignment and turbulence. These asymmetric loads contribute to extreme loads and the quantity of fatigue cycles on the rotor blades and other wind turbine components.
Various techniques have been developed to reduce fatigue caused by asymmetric loading of rotors. For example, in “Reduction of Fatigue Loads on Wind Energy Converters by Advanced Control Methods” by P. Caselitz, et al., European Wind Energy Conference, October 1997, Dublin Castle, Ireland, a technique for active damping of tower oscillations is disclosed. However, the techniques disclosed therein are based on tilt and yaw of the tower, which provides only general information about the loads on individual wind turbine components.
An additional technique is disclosed in an International Application Published under the Patent Cooperation Treaty (PCT) entitled “METHOD OF CONTROLLING THE OPERATION OF A WIND TURBINE AND WIND TURBINE FOR USE IN SAID METHOD” (Publication WO 10/33075, PCT application number PCT/DK99/00595), published May 10, 2001. The PCT application discloses a turbine controller that controls the pitch of wind turbine blades based on the mechanical loads placed on the blades. However, the control system of the PCT application is complex because multiple blade sensor outputs must be evaluated constantly.